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Hawaiian Islands Synoptic Discussion and Guidance

997
FXHW60 PHFO 040620
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
820 PM HST Fri Jul 3 2020

.SYNOPSIS...
Light to moderate east-southeast winds will continue through the
holiday as a weak trough moves through. A modest increase in
moisture associated with this trough combined with a weak upper
disturbance will be enough to support the showery conditions
continuing. Although most of the shower coverage will focus over
windward and mountain locations, some leeward showers will be
possible, especially where afternoon sea breezes develop. A
return of breezy trade winds and drier conditions is anticipated
Sunday into next week.

&&

.DISCUSSION...
Latest observations and satellite data show a weak trough moving
through the islands from east to west - somewhere in the vicinity
of Maui County and Oahu this evening. The low-level flow has
responded and shifted out of the east-southeast over the eastern
end of the state and out of the east-northeast over the western
end. Trade wind convergence into this trough combined with a weak
mid- to upper- level trough and a sufficient amount of low-level
moisture support the showery conditions in place for portions of
the islands (mostly windward areas). Satellite-estimated
precipitable water (PW) imagery reflects this and depicts a decent
sized area of higher than normal moisture in the area with values
ranging from 1.5 to 1.6 inches. The latest rainfall summary
showed peak 6-Hr (through 6 PM HST) accumulations ranging from
half of an inch to an inch (peak observed was at Lanai City with
1.15 inches).

Guidance remains in decent agreement and shows this weak surface
trough continuing westward Saturday over the western end of the
state, then west of the islands Sunday. As a result, the moist
and showery conditions will hold through the first half of the
weekend. Although most of the shower coverage will focus over
windward and mountain locations, some leeward showers will remain
possible, especially where afternoon sea breezes develop
Saturday. Some showers may be briefly heavy in some locations.

A return of breezy trade winds and drier conditions is
anticipated Sunday into early next week as high pressure builds
north-northeast of the state.

&&

.AVIATION...
A light to moderate easterly wind flow lies over the main
Hawaiian Islands. A weak low level north to south oriented trough,
currently in Maui County, will be moving westward, passing Oahu
later tonight, and Kauai Saturday morning. Lagging a few miles
behind the trough axis is an area of instability, high humidity,
and scattered showers. The current tops of the showers, between 8
to 10k feet, will be rising to as high as 15k feet by Saturday
afternoon.

Satellite infrared imagery shows broken to overcast coverage over
the leeward waters extending from the Kona coast to the Kauai
Channel. There is a bit less coverage across the windward waters
except for the Big Island east facing shores from Cape Kumukahi to
South Point, where there is an area of overcast clouds banked up
against the shoreline. This area will be monitored closely for it
may flare up in the coming hours as it moves onshore leading to a
AIRMET Sierra for mountain obscuration. The Kona coast will also
observe a shower or two along the coastline the rest of tonight.
Elsewhere, scattered showers will be favoring the windward and
mountain areas, causing brief periods of MVFR ceilings and vis.

&&

.MARINE...
A surface high pressure system is evident far north-northeast of
the main Hawaiian Islands. However, a weak surface trough, which
appears to be near Maui County early this evening, continues to
disrupt the pressure gradient over the islands as it moves slowly
westward. This will likely maintain light to moderate trades over
the state this weekend. After the surface trough departs the
region, the forecast guidance indicates surface high pressure
will build far north of the islands early next week. This will
cause the the trade winds to gradually strengthen, and Small
Craft Advisory (SCA) conditions will likely develop across most
of the typically windier waters adjacent to the islands of Maui
County and the Big Island starting Monday or Tuesday.

Surf is expected to remain below the High Surf Advisory criteria
along all shorelines through early next week. The current small
south swell will gradually decline this weekend. Otherwise,
expect a mix of small background swells from the south-southeast
and south-southwest through next Tuesday. A new longer period
south-southwest swell arriving next Wednesday is expected to
gradually build through Thursday. This may cause a slight
increase in surf along most south facing shores by the middle of
next week.

The weak trade wind regime will keep surf small along east facing
shores into early next week. As the trade winds strengthen,
expect choppy surf to increase along east facing shores from late
Tuesday through the end of next week. Surf along most north facing
shores will remain nearly flat for the foreseeable future.
However, small background easterly trade wind wrap and small
northwest swell energy may possibly produce periods of tiny surf
along some north facing beaches and reefs.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Gibbs
AVIATION...H Lau
MARINE...Houston

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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